Generally, NOx (Nitrogen Oxides) contained in exhaust gas of an internal combustion engine is purified in reaction of the NOx with a reducing agent in the presence of a reducing catalyst disposed inside an exhaust passage. For example, a Patent Literature (JP 2009-162173 A) discloses a purifying system that reforms a reducing agent by partially oxidizing the reducing agent through a plasma discharging process. The purifying system supplies the reducing agent that has an enhanced reducing capacity by the reformation into an exhaust passage at a position upstream of a reducing catalyst. By reforming the reducing agent, a NOx purification rate may be improved.